---
title: "Reference: Answer Similarity Scorer | Evals"
description: Documentation for the Answer Similarity Scorer in Mastra, which compares agent outputs against ground truth answers for CI/CD testing.
---

# Answer Similarity Scorer

The `createAnswerSimilarityScorer()` function creates a scorer that evaluates how similar an agent's output is to a ground truth answer. This scorer is specifically designed for CI/CD testing scenarios where you have expected answers and want to ensure consistency over time.

## Parameters

<PropertiesTable
  content={[
    {
      name: "model",
      type: "LanguageModel",
      required: true,
      description:
        "The language model used to evaluate semantic similarity between outputs and ground truth.",
    },
    {
      name: "options",
      type: "AnswerSimilarityOptions",
      required: false,
      description: "Configuration options for the scorer.",
    },
  ]}
/>

### AnswerSimilarityOptions

<PropertiesTable
  content={[
    {
      name: "requireGroundTruth",
      type: "boolean",
      required: false,
      defaultValue: "true",
      description:
        "Whether to require ground truth for evaluation. If false, missing ground truth returns score 0.",
    },
    {
      name: "semanticThreshold",
      type: "number",
      required: false,
      defaultValue: "0.8",
      description: "Weight for semantic matches vs exact matches (0-1).",
    },
    {
      name: "exactMatchBonus",
      type: "number",
      required: false,
      defaultValue: "0.2",
      description: "Additional score bonus for exact matches (0-1).",
    },
    {
      name: "missingPenalty",
      type: "number",
      required: false,
      defaultValue: "0.15",
      description: "Penalty per missing key concept from ground truth.",
    },
    {
      name: "contradictionPenalty",
      type: "number",
      required: false,
      defaultValue: "1.0",
      description:
        "Penalty for contradictory information. High value ensures wrong answers score near 0.",
    },
    {
      name: "extraInfoPenalty",
      type: "number",
      required: false,
      defaultValue: "0.05",
      description:
        "Mild penalty for extra information not present in ground truth (capped at 0.2).",
    },
    {
      name: "scale",
      type: "number",
      required: false,
      defaultValue: "1",
      description: "Score scaling factor.",
    },
  ]}
/>

This function returns an instance of the MastraScorer class. The `.run()` method accepts the same input as other scorers (see the [MastraScorer reference](./mastra-scorer)), but **requires ground truth** to be provided in the run object.

## .run() Returns

<PropertiesTable
  content={[
    {
      name: "runId",
      type: "string",
      description: "The id of the run (optional).",
    },
    {
      name: "score",
      type: "number",
      description:
        "Similarity score between 0-1 (or 0-scale if custom scale used). Higher scores indicate better similarity to ground truth.",
    },
    {
      name: "reason",
      type: "string",
      description:
        "Human-readable explanation of the score with actionable feedback.",
    },
    {
      name: "preprocessStepResult",
      type: "object",
      description: "Extracted semantic units from output and ground truth.",
    },
    {
      name: "analyzeStepResult",
      type: "object",
      description:
        "Detailed analysis of matches, contradictions, and extra information.",
    },
    {
      name: "preprocessPrompt",
      type: "string",
      description: "The prompt used for semantic unit extraction.",
    },
    {
      name: "analyzePrompt",
      type: "string",
      description: "The prompt used for similarity analysis.",
    },
    {
      name: "generateReasonPrompt",
      type: "string",
      description: "The prompt used for generating the explanation.",
    },
  ]}
/>

## Scoring Details

The scorer uses a multi-step process:

1. **Extract**: Breaks down output and ground truth into semantic units
2. **Analyze**: Compares units and identifies matches, contradictions, and gaps
3. **Score**: Calculates weighted similarity with penalties for contradictions
4. **Reason**: Generates human-readable explanation

Score calculation: `max(0, base_score - contradiction_penalty - missing_penalty - extra_info_penalty) × scale`

## Example

Evaluate agent responses for similarity to ground truth across different scenarios:

```typescript title="src/example-answer-similarity.ts" showLineNumbers copy
import { runEvals } from "@mastra/core/evals";
import { createAnswerSimilarityScorer } from "@mastra/evals/scorers/prebuilt";
import { myAgent } from "./agent";

const scorer = createAnswerSimilarityScorer({ model: "openai/gpt-4o" });

const result = await runEvals({
  data: [
    {
      input: "What is 2+2?",
      groundTruth: "4",
    },
    {
      input: "What is the capital of France?",
      groundTruth: "The capital of France is Paris",
    },
    {
      input: "What are the primary colors?",
      groundTruth: "The primary colors are red, blue, and yellow",
    },
  ],
  scorers: [scorer],
  target: myAgent,
  onItemComplete: ({ scorerResults }) => {
    console.log({ 
      score: scorerResults[scorer.id].score,
      reason: scorerResults[scorer.id].reason,
    });
  },
});

console.log(result.scores);
```

For more details on `runEvals`, see the [runEvals reference](/reference/v1/evals/run-evals).

To add this scorer to an agent, see the [Scorers overview](/docs/v1/evals/overview#adding-scorers-to-agents) guide.
